As a current-type light-emitting device, an Organic Light-Emitting Diode (OLED) has been more and more used in a display apparatus with high performance. A traditional Passive Matrix OLED requires a shorter driving time for a single pixel with the increasing of display size, thereby a transient current should be increased and power consumption increases. Also, an application of a great current may lead to an over-large voltage drop on wires of nanometer Indium Tin Oxides (ITO), and cause an over-high operation voltage of the OLED, which may in turn decrease its efficiency. As compared, an Active Matrix OLED (AMOLED) may settle these problems perfectively by scanning input OLED currents progressively by means of switch transistors.
In a design for a backboard of the AMOLED, a major problem needed to be settled is the non-uniformity in brightness among pixel unit circuits.
Firstly, the AMOLED constructs the pixel unit circuit with Thin-Film Transistors (TFTs) to provide corresponding currents for OLED devices. In the prior art, Low Temperature Poly-Silicon TFTs (LTPS TFTs) or Oxide TFTs are mostly used. As compared with a general amorphous-Si TFT, the LTPS TFT and the Oxide TFT have higher mobility and more stable performance, and are more suitable to be applied to the AMOLED display. However, the LTPS TFT manufactured on a glass substrate with a large area often has non-uniformity in electrical parameters such as a threshold voltage, the mobility, and the like because of the limitation on a crystallization process. Such non-uniformity may be transformed as a current difference and a brightness difference of the OLED display devices, and be perceived by human eyes, that is, a Mura phenomenon. The Oxide TFT has a good uniformity in the process, but like a-Si TFT, its threshold voltage would drift under a long time pressure and a high temperature and amounts of the drift in the thresholds of the TFTs in respective parts on a panel would be different because displayed pictures are different, which may lead to differences in the display brightness. Because such difference relates to images displayed previously, it is generally shown as an image sticking phenomenon.
Secondly, in the display application with a large size, because power lines of the backboard have certain resistances and the driving current for all pixels are provided by an ARVDD power supply, a power supply voltage at a region close to a supply position of the ARVDD power supply is higher than that at a region far away from the supply position in the backboard, and such phenomenon is called as voltage drop of power supply (IR Drop). The IR Drop may also lead to the current differences among the different regions and then generate the Mura phenomenon when displaying, since the voltage of the ARVDD power supply is associated with the current. The LTPS process constructing the pixel unit with P-Type TFTs is especially sensitive to this problem, because the storage capacitor thereof is connected between the ARVDD power supply and gates of the TFTs, and a voltage Vgs at the gate of the TFT would be affected directly when the voltage of the ARVDD power supply changes.
Thirdly, the OLED device may also cause the non-uniformity in the electric performance because of the non-uniformity in thicknesses of the films in evaporation. For the a-Si or Oxide TFT process constructing the pixel unit with N-Type TFTs, the storage capacitor thereof is connected between a gate of a driving TFT and an anode of the OLED, and the gate voltages Vgs applied actually to the TFTs would be different if the voltages at the anodes of the OLEDs for respective pixels are different when a data voltage is transferred to the gates, such that the different driving currents may cause the difference in the display brightness.
The AMOLED may be classified into three major classes based on the driving types: a digital type, a current type, and a voltage type. Herein, the digital type driving method realizes gray scales by a manner of controlling driving timing with the TFTs served as switches without compensating for the non-uniformity, but its operation frequency would increase doubled and redoubled as the display size grows, which leads to a great power consumption, and reach a physical limitation of the design within a certain range, therefore it is not suitable for the display application with the large size. The current type driving method realizes the gray scales by a manner of providing directly the driving transistors with currents having different values, and may compensate for the non-uniformity of the TFT and the IR drop better, but when a signal having a low gray scale is written, an over-long write time may be raised because a small current charges a big parasitic capacitor on the data lines, such problem is especially severe and cannot be overcome in the display with the large size. The voltage type driving method is similar to a driving method for the traditional Active Matrix Liquid Crystal Display (AMLCD) and provides a voltage signal representing the gray scale by a driving IC, and the voltage signal may be transformed to a current signal of the driving transistor inside the pixel circuit so as to drive the OLED to realize the luminance gray scales. Such method has advantages of a quick driving speed and simple implementation, which is suitable for driving the panel with the large size and widely used in industry, however it need to design additional TFTs and capacitor devices to compensate for the non-uniformity of the TFTs, the IR Drop and the non-uniformity of the OLED.
FIG. 1 illustrates a typical pixel unit circuit in the prior art. As illustrated in FIG. 1, the typical pixel unit circuit comprises two thin film transistors T2 and T1, and one capacitor C. This circuit is a typical structure for a pixel circuit of a voltage driving type (2T1C). Herein the thin film transistor T2 operates as a switch transistor, transfers a voltage on a data line to the gate of the thin film transistor T1, which operates as a driving transistor, and the driving transistor transforms the data voltage to a corresponding current to be supplied to an OLED device. The driving transistor T1 should be in a saturation zone when it operate normally, and provide a constant current during a scanning period of time for one row. The current may be expressed as follows:
      I    OLED    =            1      2        ⁢                  μ        n            ·              C        OX            ·              W        L            ·                                    (                                          V                data                            -                              V                OLED                            -                              V                thn                                      )                    2                .            
where μn is a mobility of carriers, COX is a capacitance in an oxide layer at a gate,
  W  L    is a width-length ratio of the transistor, Vdata is a signal voltage on the data line, VOLED is an operation voltage of the OLED and is shared by all pixel unit circuits, Vthn is a threshold voltage of the TFT, which is a positive value for an enhanced TFT and is a negative value for a depletion TFT. It can be seen from the above equation that the currents would be different if the Vthn is different among the different pixel units. If the Vthn of a pixel drifts with time, the currents before and after drifting would be different, which results in the image sticking. Also, the difference in the current is also caused by difference in the operation voltage of the OLEDs because of non-uniformity in the OLED devices.
There are many pixel structures for compensating for the non-uniformity of Vthn, the drifting and the non-uniformity of the OLEDs, and they are divided into two classes, an internal compensation and an external compensation, generally. Herein, a major difficulty in the designing of the external compensation is a current sensing circuit, and the pixels of each column (Pixel) in the display panel (PANEL) correspond to, generally, one sensing circuit unit, respectively, in order to increasing a read speed. A major function of the sensing circuit is to convert the current output or input into a voltage signal, which is transferred to the subsequent ADC module to be further processed. A traditional sensing circuit is composed of current integrators, and the converted output voltage is associated with an offset voltage of an amplifier. The offset voltage of the amplifier in each of the sensing circuit units is different from each other generally because of process errors and system errors, therefore an accuracy of the output voltage may decrease, such that the differences in the currents among the respective columns in the display panel can not be compared exactly.
In order to settle the above problems, the invention has made a beneficial improvement.